The present invention relates to the glass frits which, screen-printed on their own, in an overlayer or in an enamel, are capable of stopping the heat-activated migration of Ag.sup.+ ions between a support of float glass and a silver-based resistive layer.
Enamel compositions for glass are well known in the art. They are generally formed after firing by a vitreous phase supplied by the frit(s) used and a coloring and opacifying crystalline pigment phase to which other compounds may be added to obtain various effects. They are suitable for various applications and may be used inter alia for forming borders at the periphery of sheets of flat float glass which are thermoformed and then tempered for use as glazing in motor vehicles.
Originally used with the sole object of protecting the adhesive used to fix the sheets of glass (windshields, rear windows, side windows) to bodywork against the ultraviolet radiation in sunlight, these strips of black or gray enamels were subsequently the subject of aesthetic research. Attempts have been made in particular to use these enamels to conceal the lateral silver conducting tracks of heated rear windows and the welding spots of the connecting terminals of the current supply wires. It has now been found that after the heat treatment required for curving and tempering, none of the enamels of conventional compositions are capable of completely concealing the silver conducting tracks applied in superposed form and heated with the enamel or the above-mentioned weld spots.
Various studies have demonstrated the existence of heat-activated transport of the Ag.sup.+ cations from the resistive layer through the enamel to the enamel/glass interface, thus revealing the presence of conducting tracks behind the enamel by an intense colorimetric contrast.
It is known from U.S. Pat. No. 4,837,383 that it is desirable to prevent the migration of silver when using an enamel consisting of a glass flux based on a glass frit, pigments, elemental aluminum and/or lithium oxide and a vehicle for paste formation. European patent application EP-A 0 377 062 claims improvements in the silver-based opacity of enamels by the addition before paste formation of silicon, boron, carbon, lead and/or silver in their elemental forms.
It is known from U.S. Pat. No. 5,141,798 that heavy metal sulfides can be used in an enamel for coating onto glass panes with silver conducting layers. The enamel contains glass frit and pigments in the form of heavy metal oxides or heavy metal sulfides. However, the sulfides are not part of the glass frit.
Additions of the type mentioned have not proved entirely satisfactory for obvious reasons of cost (silver), special operating conditions necessitated by the introduction of highly reducing elements, such as silicon or boron, stability to acids (lithium) or toxicity (lead). The enamels developed with additions such as these in conjunction with the present state-of-the-art in this field show that, despite the aesthetic interest there is in completely concealing the silver conducting tracks by enamel, such solutions limit the range of colors currently useable by the automotive industry to a few blacks and to a few grays.
In addition, the quality of the masking effect obtained is largely influenced by the firing cycle as a whole.